// Copyright 2014 The Chromium Authors. All rights reserved.
// Use of this source code is governed by a BSD-style license that can be
// found in the LICENSE file.

#ifndef UI_EVENTS_GESTURE_DETECTION_MOTION_EVENT_BUFFER_H_
#define UI_EVENTS_GESTURE_DETECTION_MOTION_EVENT_BUFFER_H_

#include "base/macros.h"
#include "base/memory/scoped_ptr.h"
#include "base/memory/scoped_vector.h"
#include "base/time/time.h"
#include "ui/events/gesture_detection/gesture_detection_export.h"

namespace ui {

class MotionEvent;
class MotionEventGeneric;

// Allows event forwarding and flush requests from a |MotionEventBuffer|.
class MotionEventBufferClient {
public:
    virtual ~MotionEventBufferClient() { }
    virtual void ForwardMotionEvent(const MotionEvent& event) = 0;
    virtual void SetNeedsFlush() = 0;
};

// Utility class for buffering streamed MotionEventVector until a given flush.
// Events that can be combined will remain buffered, and depending on the flush
// time and buffered events, a resampled event with history will be synthesized.
// The primary purpose of this class is to ensure a smooth output motion signal
// by resampling a discrete input signal that may run on a different frequency
// or lack alignment with the output display signal.
// Note that this class is largely based on code from Android's existing touch
// pipeline (in particular, logic from ImageTransport, http://goo.gl/Ixsb0D).
// See the design doc at http://goo.gl/MdmpCf for more details.
class GESTURE_DETECTION_EXPORT MotionEventBuffer {
public:
    // The provided |client| must not be null, and |enable_resampling| determines
    // resampling behavior (see |resample_|).
    MotionEventBuffer(MotionEventBufferClient* client, bool enable_resampling);
    ~MotionEventBuffer();

    // Should be called upon receipt of an event from the platform, prior to event
    // dispatch to UI or content components. Events that can be coalesced will
    // remain buffered until the next |Flush()|, while other events will be
    // forwarded immediately (incidentally flushing currently buffered events).
    void OnMotionEvent(const MotionEvent& event);

    // Forward any buffered events, resampling if necessary (see |resample_|)
    // according to the provided |frame_time|. This should be called in response
    // to |SetNeedsFlush()| calls on the client. If the buffer is empty, no
    // events will be forwarded, and if another flush is necessary it will be
    // requested.
    void Flush(base::TimeTicks frame_time);

private:
    typedef ScopedVector<MotionEventGeneric> MotionEventVector;

    void FlushWithResampling(MotionEventVector events,
        base::TimeTicks resample_time);
    void FlushWithoutResampling(MotionEventVector events);

    MotionEventBufferClient* const client_;
    MotionEventVector buffered_events_;

    // Time of the most recently extrapolated event. This will be 0 if the
    // last sent event was not extrapolated. Used internally to guard against
    // conflicts between events received from the platform that may have an
    // earlier timestamp than that synthesized at the latest resample.
    base::TimeTicks last_extrapolated_event_time_;

    // Whether buffered events should be resampled upon |Flush()|. If true, short
    // horizon interpolation/extrapolation will be used to synthesize the
    // forwarded event. Otherwise the most recently buffered event will be
    // forwarded, with preceding events as historical entries. Defaults to true.
    bool resample_;

    DISALLOW_COPY_AND_ASSIGN(MotionEventBuffer);
};

} // namespace ui

#endif // UI_EVENTS_GESTURE_DETECTION_MOTION_EVENT_BUFFER_H_
